1. Field of the Invention
The present invention relates to an electrode used for electrochemical devices such as batteries or capacitors, and a battery using the same.
2. Description of the Related Art
In electrochemical devices such as batteries or capacitors, various examinations for a low cost production have been continued. For example, in case-of alkaline storage batteries, which utilize nickel hydroxide that is a kind of nickel oxide as a positive electrode, there has been widely employed a non-sintered type nickel electrode. The non-sintered type nickel electrode can be formed in such a manner that the nickel hydroxide is dispersed into water or a solvent with a binder or a viscosity thickening agent and the like to form a paste state and then the paste is to be applied on and filled into a conductive porous substrate such as a foamed nickel and the like. Further, in case of the non-sintered type nickel electrode with utilization of the porous substrate, there has been commonly employed poly-tetra-fluoroethylene (PTFE) as a binder thereof. This process requires a stable compound that is hardly oxidized as a binder, since the nickel electrode is over charged with an alkaline solution in a battery and exposed at an oxygen gas atmosphere. The PTFE best satisfies such a requirement.
In case of the non-sintered type nickel electrode, however, there is a problem such as a high cost in production although the non-sintered nickel electrode has an advantage in which the filling density of nickel hydroxide as an active material is enhanced and it is also easy to produce tile electrode. The high cost problem is caused by processes to manufacture the porous substrate such as a foamed nickel and the like in which urethane foam is commonly coated with metal such as nickel and the like, sintered at a reduction atmosphere, and removed.
Thus, it is suggested that a substrate, in which metal fiber and the like are put on the upper and lower surfaces of a metal plate, be used as a collector (refer to Japanese Unexamined Patent Application Publication No. H 09-265991). And it is also suggested that a metal plate, which is, compressed with an embossing process and provided with concave-convex parts, be used as a substrate (refer to Japanese Unexamined Patent Application Publication No. H 10-106580)
However, in case of utilizing the substrate provided with concave-convex parts made by an embossing process, the cost for producing a positive electrode may be reduces, but a binder of PTFE causes such a problem that the positive electrode mixture is easy to fall off from the substrate. The reason is as below. When the PTFE is compressed, it tends to be linearly extended. Therefore, if processes such as pressing, cutting off and the like are performed, the positive electrode mixture may slide off from the substrate, resulting in easy peeling off and falling off of the mixture therefrom. Further, when a battery is charged and discharged, the active material is repeatedly expanded and contracted, resulting in easy peeling off of the positive electrode mixture from the substrate and then increasing of the contact resistance between the substrate and the positive electrode mixture. As a result, the battery is difficult to be charged as well as generation of oxygen gas becomes increased during charging. Furthermore, as a result, the battery cannot obtain a sufficient discharge capacity, resulting in early deterioration in cycle. To solve such problems, other binders have been examined, but binders having any good characteristics have not been realized yet.
Further, there has been another problem that the above substrate, compared to a conventional porous substrate, has bad current-collection performance and cannot enhance the coefficient of utilization of a positive electrode.
Further, such a surface-processed substrate is easy to be cut off due to doctor blades when continuously applying. Or, there is a problem that it is difficult to make a uniform coating application on the substrate. Further, parts of the substrate tends to be exposed at a surface of the resulting electrode after molding, resulting in cause of short. Furthermore, if intervals between doctor blades keep broaden to avoid the above problem, it causes problem center protrusion of the substrate becomes difficult, resulting in difficulty of uniform coating on both surfaces of the substrate. The amount of active materials falling off from the substrate increases, thereby it is difficult to collect current and decreases the coefficient of utilization.